Hematopoiesis is a regulated developmental process that generates the cells of the blood and lymph. Transcription factors are key proteins central to the regulation of the differentiation of the hematopoietic stem cell into the various mature cell types. The specific transcription factor PU.1 is known to be necessary for lymphoid and myeloid development. By generating an inducible PU.1 protein, one can investigate the role of PU.1 in the development and homeostasis of the hematopoietic lineages. Understanding the molecular control of stem cell biology will be clinically significant for the treatment of hematological and oncological diseases, aid in bone marrow transplantation, and will be central to the future of gene therapy. Previously, it has been shown that PU.1 -/- mice die in utero with no detectable lymphoid or myeloid cells. Therefore, PU.1 is necessary early in the development of these lineages. PU.1 is also expressed in mature B cells and macrophages. A novel approach has been devised to look at PU.1 function later in the development of these lineages. The PU.1 cDNA has been fused to a mutant version of the hormone binding domain of the estrogen receptor. This fusion protein is in an inactive state, and will only become active by the addition of the synthetic hormone tamoxifen. The tamoxifen responsive fusion protein is refractory to native estrogens, thus facilitating both in vitro and in vivo modulation of PU.1 function. Animals will be generated utilizing the property of homologous recombination to "knock-in" this fusion protein into the endogenous PU.1 locus. Mice will develop in the presence of tamoxifen, and thus PU.1 will be able to function normally. By removing tamoxifen, one can investigate the loss of a functional PU.1 protein in adult hematopoiesis and mature cells.